Different types of memory devices have been proposed in the last years. For example, a phase-change E2PROM is a non-volatile memory exploiting the properties of a material that can be reversibly switched between an amorphous phase and a crystalline phase. The phase-change material exhibits different electrical characteristics depending on its phase, each one representing a corresponding logic value. An example of a phase-change E2PROM is described in U.S. Pat. No. 5,166,758.
Typically, the memory device includes a matrix of memory cells, each one consisting of a storage element connected in series to an access selector. During a reading or programming operation on the memory device, the selectors are suitable biased so as to allow accessing the corresponding storage elements.
A solution known in the art for implementing those selectors is that of using bipolar elements; for example, each bipolar element can be a diode (as described in US-A-2002/0079524) or a base/emitter junction of a BJT transistor. In this case, the bipolar element is forward biased when selected (while it is reverse biased otherwise).
A drawback of the solution described above is that the bipolar element exhibits a high leakage current when reverse biased. Moreover, the bipolar element involves a high voltage drop when forward biased (for example, 0.6 V); this voltage drop adversely affects the accuracy of the reading operation.
In order to avoid the drawbacks referred to above, the selectors are commonly implemented with MOS transistors (and especially NMOS transistors). However, in this case each NMOS transistor must be quite wide in order to sustain the high current that necessarily flows through the corresponding storage element during a programming operation. This causes a waste of area in a chip of semiconductor material wherein the memory device is typically integrated.